The invention relates to temperature compensation of a voltage controlled crystal oscillator (VCXO), and in particular, to a method and apparatus for adaptive temperature compensation of a VCXO used in a mobile communications terminal.
The reference frequency in a mobile station is generated with the aid of a crystal. The crystal is needed in order to fulfil frequency stability and accuracy requirements, as imposed by telecommunication standards such as GSM.
Unfortunately, the stability and accuracy of the frequency is temperature dependent.
While this can be a disadvantage, it does not pose any problems while the mobile station is in communication with a base station, since it can control the reference frequency using frequency correction information transmitted in the downlink signal from the base station.
However, in a GSM system, during temporary loss of the signal received from the base station, the mobile station must still be required to maintain its reference frequency to an accuracy of 0.2 ppm during a period of 64 slow associated control channel (SACCH) blocks.
Thus, if the temperature changes during the gap in reception, conventional non-temperature compensated crystals will not be able to fulfil the accuracy requirements mentioned above.
The worst case scenario is when the mobile station starts to transmit on the highest output power and then loses its synchronisation with the base station. The temperature change in the mobile station, due to heating of the power amplifier, will raise the temperature of the crystal. This change in crystal temperature will cause the frequency to drift.
Many solutions are known to exist to overcome this problem.
According to one solution, an average temperature dependency curve is supplied by the manufacturer of the crystal. The crystals are screened according to how much they deviate from the average temperature curve. Only crystals that fall within acceptable limits from the average temperature curve are thereafter used in manufacture. When the mobile station is in use, it measures the temperature and uses the average temperature dependancy curve to compensate the tuning of the reference frequency in the mobile station.
This solution has many disadvantages. For example, manufacturers have to screen crystals which deviate too much from the average temperature dependancy curve. This results in a low yield, particularly for applications having a large temperature gradient. This problem becomes more relevant as mobile stations become smaller, resulting in greater temperature gradients within the mobile stations. In addition, the size of the crystal also dictates the temperature gradient which, again, also increases as the size of the package reduces.
FIG. 1 shows another commonly known solution in which a crystal 1 is packaged with a reference voltage controlled oscillator VCO 3, (forming a voltage controlled crystal oscillator VCXO 4), a thermistor 5, an analogue to digital converter (ADC) 7, control logic 9, and a digital to analogue converter (DAC) 11. This type of module, often called a temperature compensated crystal oscillator (TCXO) 13, is cycled in temperature when it is manufactured. This is done in order to make an individual trimming over the entire working temperature range of the crystal 1.
The thermistor 5 measures the temperature of the crystal 1, and its output signal 15 is converted by the ADC 5 into a digital signal 17 for processing by the control logic 9. Depending on the measured temperature, the control logic 9 outputs a corresponding digital value 19, or compensation value, to the DAC 11 which provides a tuning signal 21 to the VCO 3 to obtain the desired reference frequency Fref.
This type of solution suffers from the disadvantage of having to cycle each individual crystal though the entire working temperature range during manufacture, which takes an unreasonable amount of time. In addition, the provision of the thermistor 5, ADC 7, control logic 9 and DAC 11 all add to the overall cost of the crystal 1.
Furthermore, the solution of FIG. 1 must either store a vast number of temperature values and corresponding DAC values (which in turn requires a large memory), or store fewer temperature values, which requires the use of a complex interpolation technique or algorithm to deduce a DAC value from a temperature dependancy curve.
The aim of the present invention is to provide a method and apparatus for adaptive temperature compensation of a voltage controlled crystal oscillator which does not suffer from the disadvantages mentioned above.
According to a first aspect of the present invention, there is provided a method of controlling the output frequency of a voltage controlled crystal oscillator (VCXO) which is controlled by a VCXO control signal, the method comprising the steps of:
(a) during a first period, receiving frequency correction information from a first source, and using this frequency correction information to control the output frequency of the VCXO; characterised by the steps of
(b) during the first period, determining a plurality of VCXO compensation values which correspond to respective predefined temperature regions, and which are indicative of a linear relationship between the VCXO control signal and temperature in the region concerned;
(c) storing the compensation values; and,
(d) during a second period when the frequency correction information from the first source is not available, determining the temperature range in which the VCXO is operating and using the corresponding stored compensation value to determine the VCXO control signal, thereby controlling the output frequency of the
According to another aspect of the invention, there is provided an apparatus for controlling the output frequency of a voltage controlled crystal oscillator (VCXO) which is controlled by a VCXO control signal, the apparatus comprising:
(a) means for receiving frequency correction information from a first source during a first period;
(b) means for using the frequency control information to control the output frequency of the VCXO during the first period; characterised in that the apparatus further comprises:
(c) means for determining a plurality of VCXO compensation values during the first period, each VCXO compensation value corresponding to a respective predefined temperature region, and indicative of a linear relationship between the VCXO control signal and temperature in the region concerned;
(d) a first memory for storing the compensation values; and,
(e) means for determining the temperature range in which the VCXO is operating, and using the corresponding stored compensation value to determine the VCXO control signal for controlling the output frequency of the VCXO during a second period when the frequency correction information is not available.
According to yet another aspect of the present invention, there is provided a mobile communications terminal comprising;
a voltage controlled crystal oscillator, VCXO (25);
a digital to analogue converter, VCXO-DAC, (29) for controlling the VCXO (25) according to a VCXO control signal (39);
means (37) for receiving frequency correction information from a base station (33);
temperature measuring means (43);
an analogue to digital converter (45) for converting the temperature signals into digital signals;
control means (31) for controlling the output frequency of the VCXO 25; wherein the control means comprises:
means for using the received frequency control information to control the output frequency of the VCXO during a first period; characterised in that the mobile communications terminal further comprises:
means for determining a plurality of VCXO compensation values during the first period, each VCXO compensation value corresponding to a respective predefined temperature region, and indicative of a linear relationship between the VCXO control signal and temperature in the region concerned;
a first memory (41) for storing the compensation values; and,
means for determining the temperature range in which the VCXO is operating, and using the corresponding stored compensation value to determine the VCXO control signal for controlling the output frequency of the VCXO during a second period when the frequency correction information is not available.